YS120 - air in fuel line.

[malc1]

I have air bubbles in the clear tube between the regulator and the carb body.

Basically this is giving me a very rough idle and where ever i have the idle needle doesn't really make much difference.
This Idle is very bumpy with misfiring and loads of vibration felt.

Idle needle is set at just less than 1 turn out and has been as low as 1/2 turn while adjusting on the bench,

I have just modified the fuel tank by fitting Dubro tank bungs and replaced all the fuel and clunk line and I'm using an OS bubble less clunk and a bubbleless fuel filter.


Plug has been changed and I'm using 20% Optifuel.

There is a lot of swirling fuel at the top of the tank when running so pressure must be there.

Any ideas before I pull the engine out again.

[Glenn Goodlett]

I once had a pin hole in the line between the carb and reg that gave me lots of problems. Maybe replace that.

[malc1]

Engine has now been removed to be checked.

First thing I have noticed is that the motor was absolutely fuel of unburnt fuel.
I had it running out of the carb,the exhaust,back plate pressure nipple and when I took the back of it just poured out.
So much fuel for just attempting to get it to idle.

Seems strange this really.

I'm going to strip the regulator later and as I have one spare swap the diaphragm.
The oneway valve looks Ok.

[crashr30]

I will readily admit that I may not know what I am talking about here, but when you remove the regulator and/or carb look for a small pin(brass) that is in the crankcase. It is supposed to be in the case under the carb insulator or the regulator, not sure which. It is my understanding that the pin can fracture and generate an air leak. Sealing that hole is supposed to address this sort of issue.

I heard this from a top pilot but I do not own a 120 nor have I taken one apart. So if this helps great and if not then sorry for spreading rumors.

Good luck.

[sagiadir]

i have the same problem and I found that the fuel line was bad....
the one between the regulator and the engine.....
I just crash my 700 because a lot of vibration from the engine....
now its time to rebuild it hopefully it will help.....

[malc1]

Diaphragm has now been replaced just in case.
Idle needle has also been reduced by 1/4 of a turn to 3/4 out.

There are still air bubbles in that short length of tubing though with the engine idling and can't see any air leaks now.

Test flown on Saturday and found it running overall only a little bit better but still not right.Pretty rough actually.

Going to try an Enya No5 plug at the weekend instead of the YS no2 that I am using at the moment to see if that makes it run any better.

[ChapaWolf]

I can think of one thing that would likely cause that and its the fuel filter. Make absolutely sure you have a good seal at the fuel filter. What I am referring to is that o-ring on the filter that seals the two halves. It will suck air way before it leaks fuel. I always take two pairs of pliers and tightened the heck out of it.

If that doesn't work then remove the entire fuel system, the pressure line to the tank the tank the header tank the lines all the way to the engine. When removed assemble them exactly how they are on the chopper plug the end going to the carb. Fill a bucket up with water and get your self a basket ball air pump the one with the plunger. It doesn't have to be exactly a basket ball pump but you need something to pressurize the system. Once you have all that you need to hold the entire system under water in the bucket and pressurize it to at least the operating pressure. Which ever part is sucking air will be obvious by the bubbles coming out. I actually started to do this whenever I replace any tubing on the fuel system. Its better to know for sure if the system is air tight before flying for me any ways.

I do about six or seven pumps with a basket ball pump. You can see its pressurized as soon as the tank begins to bulge and fell hard.

This is cheap and easy insurance for making sure your not chasing needles all day.